1. Field of the Invention
The present invention relates to ion beam delivery equipment and ion beam delivery method, which are used to produce and deliver ion beam, e.g., proton or carbon ions, to a tumor for treatment.
2. Description of the Related Art
There is known a method for delivering ion beam, e.g., proton or carbon ions, to a tumor, such as a cancer, in the body of a patient. The ion beam delivery equipment for such treatment comprises an ion beam generator to produce the said ion beam and accelerate it to a needed energy, a beam transport system, and an beam delivery nozzle. An ion beam accelerated by the beam generator reaches the beam delivery nozzle, which is installed in a rotating gantry to monitor and shape the therapeutic radiation field, through a first beam transport system and a second ion beam transport system, the latter being installed in the rotating gantry. The ion beam reached the beam delivery nozzle is delivered to the tumor in the patient body from the beam delivery nozzle. Known examples of the beam generator include a synchrotron (quasi-circular accelerator) provided with an extraction deflector for extracting the ion beam from the orbit (see, e.g., patent Reference 1; U.S. Pat. No. 5,363,008).
In radiation therapy using an ion beam, e.g., with a proton beam delivering a radiation dosage to a tumor, by utilizing characteristics that most of the energy of the proton beam is released just before protons come to rest, namely that a Bragg peak is formed just before the stop of protons, the energy of the proton beam is selected to stop protons in the tumor so that the beam energy is released most to cells within the tumor or its microscopic extensions.
Usually, the tumor has a certain thickness in the direction of depth, i.e. along the direction of the ion beam, from the body surface of a patient (hereinafter referred to simply as “the direction of depth”). To effectively irradiate the ion beam over the entire thickness of the tumor in the direction of depth, the width of the Bragg peak must be spread out in the direction of depth. The spread-out width of the Bragg peak is called a Bragg peak width. To obtain the required Bragg peak width, the energy of the ion beam must be modulated.
From that point of view, a range modulation wheel (RMW) has already been proposed in which a plurality of blades each having a thickness varied step by step in the circumferential direction are installed around a rotating shaft (see, e.g., Non-patent Reference 1; “REVIEW OF SCIENTIFIC INSTRUMENTS”, Vol. 64, No. 8, pp 2074-2084 (FIGS. 30 and 31) issued in August, 1993). The plural blades are mounted to the rotating shaft. In the RMW, a through opening is formed between adjacent sets of the blades. For example, when the RMW is rotated from a state in which the opening is positioned on a path of the ion beam (hereinafter referred to simply as a “beam path”), the opening and the blade alternately intersect the beam path. At the time when the ion beam passes through the opening, the energy of the ion beam is not attenuated and therefore the Bragg peak is produced in the deepest position inside the patient body. At the time when the ion beam passes through a blade, the energy of the ion beam is attenuated more as the ion beam passes through the blade having a larger thickness, and therefore the Bragg peak is produced in a portion of the tumor near the body surface of the patient. With the rotation of the RMW, the position in the direction of depth where the Bragg peak is formed varies cyclically. As a result, the Bragg peak width being comparatively wide and flat in the direction of depth of the tumor can be obtained, looking at the beam energy integrated over time.